Backtrack downloads have been shutdown at the official download site, it looks as though they want everyone to make the move to Kali Linux. Here is a screen shot.
It has been this way since 02-23-2014. It is sad to see Backtrack going away like this, like saying goodbye to an old friend.
I personally will continue to use Backtrack along with Kali Linux, why mess with something that works? I know updating to the latest and greatest is the way we are supposed to go, and I do use Kali Linux. But often I simply like the feel of something I know very well and as far as wireless penetration testing Backtrack has the same tools Kali Linux does.
Of course this could change in the future, but in the mean time I personally will continue to use both Backtrack 5 and Kali Linux.
I have setup a download page for the old Backtrack 5 R3 downloads. They are torrent links so I am not affiliated with them but if you need to download Backtrack 5 it looks like the best way to currently go. The link is on the top bar.